Coupling and Decoupling of Soil Carbon and Nutrient Cycles Across an Aridity Gradient in the Drylands of Northern China: Evidence From Ecoenzymatic Stoichiometry

Abstract

Abstract Drylands are characterized by stressful conditions with the limitation of both carbon (C) and nutrients, particularly nitrogen (N) and phosphorus (P). Biological C, N, and P releases from soil organic matter by enzymes are essential components for biogeochemical cycles and are sensitive to the climate in drylands. However, how the ecoenzymatic C:N:P stoichiometry responds to environmental factors (i.e., climatic and edaphic factors) over broad geographical scales remains largely unclear. We examined the patterns of ecoenzymatic C:N:P ratios across a 3,700‐km aridity gradient (0.43 < aridity < 0.97) in northern China. In wetter sites (aridity < 0.70), the relative C:N:P acquisition ratios via enzymes remained relatively constant with increasing aridity. In contrast, in drier sites (aridity > 0.70), the enzymatic C:nutrient (N and P) ratios declined as the aridity increased, while the enzymatic P:N ratios were mostly lower than those in the wetter sites. In drier sites with low C availability, the increasing carbon use efficiency and the increasing proportion of C converted to biomass (than the proportion of respiration) contributed to the declines of the enzymatic C:nutrient ratios as the aridity increased. The overall lower enzymatic P:N ratios were related to the higher soil P availability compared with N availability (higher organic P and lower soil NH 4 + :available P ratios) in drier sites. Overall, our findings indicate that intrinsic linkages of biological C, N, and P acquisitions and cycles were broken at the aridity threshold of 0.70, with higher acquisition efforts for N and P (particularly for N) with increasing aridity in drier sites with aridity > 0.70.